1. Field of the Invention
The present invention relates to a process for treating an oxygen rich exhaust gas containing nitrogen oxides, exhausted from a boiler, automobile engine, etc. with a catalyst. More specifically, it relates to a process for removing nitrogen oxides using a catalyst extremely superior durability.
The term "oxygen rich exhaust gas" used herein is intended to mean an exhaust gas containing oxygen in an amount exceeding the amount of oxygen necessary for completely oxidizing reducing agent such as carbon monoxide, hydrocarbons contained in the exhaust gas. Specific examples of such an exhaust gas include exhaust gases discharged, for example, from internal combustion engines of automobiles operating at a high air/fuel ratio (i.e., in the lean burn region).
2. Description of the Related Art
The processes for removing nitrogen oxides from an exhaust gas discharged from boilers, automobile engines, and the like, which are practically used, are the process of selective catalytic reduction using ammonia in the presence of a catalyst or the process of non-selective catalytic reduction which passes the exhaust gas through a catalyst and reduces it with the unburnt carbon monoxide and hydrocarbons.
Japanese Unexamined Patent Publication (Kokai) No. 60-125250 proposes a copper ion-exchanged zeolite as a catalyst capable of directly catalytic decomposing nitrogen oxides in the absence of a reducing agent.
For purifying the exhaust gas of diesel engines or lean burn engines designed for reduced fuel consumption, further, there has been proposed a catalyst which includes a base (or non noble) metal in the zeolite etc. as a catalyst capable of selectively reducing the nitrogen oxides by the reducing agents of the unburnt carbon monoxide, hydrocarbons, etc. even in an oxygen rich atmosphere (Japanese Unexamined Patent Publication (Kokai) No. 63-100919).
These proposed catalysts, however, suffer from problems, in particular their durability at a high temperature, and therefore, are not yet suitable for practical use.
Therefore, attempts were made to improve the durability of the zeolite in the presence of steam at high temperature by substituting the aluminum near the surface of the zeolite with silicon whereby the outer surface of the zeolite is made hydrophobic (Japanese Unexamined Patent Publication (Kokai) No. 4-271843). With this proposed process, however, ammonium silicofluoride was used for the silicon substitution in an aqueous solution, and therefore, part of the ammonium silicofluoride in the aqueous solution was hydrolyzed and produced fluoric acid. This caused dealumination in the zeolite bulk and therefore the sufficient durability could not be obtained.